This invention relates to a steel cord for use as reinforcement of tires, belts, etc.
Generally, this kind of steel cord is made by intertwisting a plurality of material wires arranged in parallel state and is covered with a rubber compound. When a steel cord is used as reinforcement, good adhesion of material wires to the rubber compound is an indispensable requirement; in other words, in order to enable a steel cord to perform its duties as a reinforcement, it is required that material wires and the rubber compound are formed into a perfectly integrated substance. In the case of tires, for example, poor adhesion of material wires to the rubber compound causes the so-called "separation phenomenon" (wire and rubber compound separate from each other) during the running of a car, with a resultant reduction of the function of the tires. Also, poor adhesion causes rusting of material wires due to moisture contained in the rubber compound and moisture in the ambient atmosphere, with the result being the deterioration of cord strength to a large extent and earlier occurrence of "separation phenomenon".
The conventional steel cord 1 is formed by twisting together a plurality of material wires 2 so that its cross section is as shown by FIG. 1(a) and FIG. 1(b), in which 2' is a core strand. When a conventional steel cord is covered with a rubber compound, the rubber compound is not allowed to penetrate into the central part of the steel cord 1 because material wires are in tight contact with each other and thus a space is left at the central part of the steel cord and good adhesion between the steel cord and the rubber compound cannot be obtained.
In order to eliminate the above disadvantage of the conventional steel cord, steel cord (3) (sometimes called "open cord") which is made by twisting together material wires, loosely with a gap left between adjoining wires as shown by FIG. 2(a) and FIG. 2(b), so that the rubber compound may penetrate into the central part of a steel cord and adhere to the whole circumference of material wires, has been taken into consideration. In this case, however, since the steel cord 3 is loose in intertwisting throughout its whole length, from its construction it has such disadvantages that material wires are one-sided as shown by FIG. 2(c), FIG. 2(d) and FIG. 2(e) or twist becomes uneven in lengthwise direction of a cord. Therefore, even if the rubber compound penetrates into the central part of a cord, the modulus of a steel cord as an integrated substance lowers as shown by FIG. 5 and also its compression flexing fatigue characteristic deteriorates. Thus, plural material wires constituting a steel cord do not display their strength as an integrated substance and one or two wires which are easier to be affected by stress break much earlier.
On the other hand, it is impossible practically to manufacture the steel cord 3 having a cross sectional shape as shown by FIG. 2(a) or FIG. 2(b) throughout its whole length, though it is possible to have such cross sectional shapes partly in the whole length of a steel cord, because material wires constituting a steel cord are not in contact with each other, in other words, must be arranged spatially.